Disturbances in the normal beating pattern of a human heart can present a serious life-threatening situation for an individual. Particularly dangerous is ventricular fibrillation in which activity within the ventricles of the heart is so uncoordinated that virtually no pumping of blood takes place. Defibrillation therapy can interrupt ventricular fibrillation and allow a normal beating pattern to be restored. Such therapy generally involves rapid delivery of a relatively large amount of electrical energy to the heart at a high voltage.
A typical defibrillator includes a pair of electrical leads for collecting electrical signals generated by the heart and for delivering defibrillation therapy. In addition, a defibrillator generally includes one or more batteries, energy storage capacitors, and control circuitry for charging the capacitors and delivering defibrillation therapy through the leads.
At present, batteries are not generally suited for directly providing the high energy, high-voltage electric pulses required for defibrillation therapy. Consequently, it is customary for defibrillators to include one or more high-voltage energy storage capacitors that are charged from a battery via appropriate charging circuitry. Once charged, the capacitors are selectively discharged by a physician or medical technician to provide the defibrillation therapy to the patient. In normal operation, the high-voltage energy storage capacitors are not maintained in a state of charge, but rather are charged immediately preceding the delivery of defibrillation therapy.
Charging circuitry in a defibrillator generally includes a switch controlled by a charge controller. At the direction of the charge controller, the switch cyclically interrupts current flow from a battery through a primary winding of a step-up transformer in order to induce a transient current in a secondary winding of the transformer during a fly-back period. The induced fly-back current in the transformer secondary winding is applied to the terminals of a high voltage energy storage capacitor, thereby causing the capacitor to charge over a number of switching cycles. Typically, the charge controller monitors both the current flowing through the primary winding and the amount of charge on the capacitor in directing the operation of the switch.
One or more batteries coupled to the defibrillator supply the energy for charging the energy storage capacitor. As a battery is depleted of its energy, electrical resistance within the battery increases. For most battery technologies, a battery experiences a gradual rise in internal resistance relative to decreasing battery terminal voltage until a point at which the internal resistance rises quickly as the battery voltage drops. Different battery technologies have different internal resistance to voltage curves.
When a defibrillator begins to charge its energy storage capacitor, a significant amount of current is drawn from the battery, causing a significant voltage drop across the internal resistance of the battery. At a certain point in operation, the internal resistance of the defibrillator battery becomes too high and the battery is unable to provide the peak current at the voltage level that the charging circuitry demands. The battery voltage collapses and drops below a power-on-reset threshold within the defibrillator causing the defibrillator to shut down, even though the battery may not be fully depleted of its energy. In other words, the battery voltage drops too low and the defibrillator, believing that the battery is fully depleted of energy, turns itself off. While the defibrillator may be turned back on, the defibrillator may be unable to deliver defibrillation therapy until a new battery is installed.
To prevent premature shutdown of a defibrillator before the battery is fully depleted, a solution has been to reduce the amount of peak current that the defibrillator charging circuitry draws from the battery when the battery terminal voltage has decreased beyond a predetermined level. At a reduced current, the battery voltage is maintained at an acceptable level and the battery is able to continue charging the defibrillator capacitor, albeit at a lower charge rate. Prior techniques for controlling the amount of current drawn from a battery include a microprocessor connected to the battery via an analog-to-digital (A/D) converter to monitor the battery voltage. When the microprocessor determines that the battery voltage has dropped below a specified threshold, the microprocessor commands the capacitor charging circuitry to draw current from the battery at a lower level. A problem inherent with this approach is that the microprocessor may be unable to react quickly enough to a significant drop in battery voltage and the defibrillator may shut down before the charging circuitry can be commanded to draw current at a lower rate.
Given the problems associated with prior art charge rate control, there is a need for capacitor charging circuitry that can quickly respond to decreasing battery voltage in a dynamic manner.